The best ways to make sure you don't plagiarize are to:
But, if you want to double-check that you've done it right once your paper is finished, there are some free plagiarism checkers available. Keep in mind that some of these sites offer limited free services...you may have to subscribe to use all features. Here are a few:
This site reviews several others: Top 10 free plagiarism detection tools for teachers.
Please note: The official plagiarism detection tool used by the University is Turnitin (read more below). The University and the Library do not endorse the free checkers linked above and cannot guarantee that their results will be accepted by your professor.